What can be responsible for subtle abortions in btrbk?

I started to do archiving daily and backups hourly and have noticed some abortions that I can't explain as all targets involved are fstab mounted and were available, here is my conf file:

```
transaction_log /var/log/btrbk.log

preserve_day_of_week sunday
timestamp_format long-iso

snapshot_preserve_min 7d

snapshot_preserve 24h 10d
target_preserve 168h 30d 12w 6m
archive_preserve 168h 30d 24w 6m

snapshot_dir snapshots

volume /mnt/FS_TREE/
subvolume root
target send-receive /mnt/HDDs/backups/snapshots/desktop
```
and here is the relevant log slice cleaned with relevant info only:

```
-------------from Cron.Hourly - Sun Jun 4 20:01:01 -03 2017 ------------
2017-06-04T20:01:11-0300 finished success 10 - - - -
-------------from Cron.Hourly - Sun Jun 4 21:01:01 -03 2017 ------------
2017-06-04T21:01:02-0300 finished partial 0 - - - # At least one backup task aborted
.
. all equaly aborted as Sun Jun 4 21:01:01 -03 2017
.
-------------from Cron.Hourly - Sun Jun 4 21:01:01 -03 2017 ------------
2017-06-05T03:35:04-0300 abort_target ABORT - /mnt/BigHDD/backups/snapshots/desktop - - # Receive targets of archive candidates exist at unexpected location
2017-06-05T03:35:04-0300 abort_archive_source ABORT - /mnt/HDDs/backups/snapshots/desktop - - # At least one target aborted earlier
2017-06-05T03:35:04-0300 finished partial 3 - - - # At least one backup task aborted
-------------from Cron.Hourly - Mon Jun 5 04:01:01 -03 2017 ------------
2017-06-05T04:01:01-0300 finished partial 0 - - - # At least one backup task aborted
.
. all equaly aborted as Mon Jun 5 04:01:01 -03 2017
.
-------------from Cron.Hourly - Mon Jun 5 09:01:01 -03 2017 ------------
2017-06-05T09:02:18-0300 finished success 76 - - - -
RETURNS TO NORMAL SNAPSHOTING
```
Another point is that there exist many codes I was not able to find a description for as "success 76 or 10".